Since the invention of laser in the 1960s, lasers have been used extensively in a variety of applications ranging from laser research, optical communications, and material processing to optical metrology. In these applications, it is very important to know the parameters that can describe the laser beam. Optical beam radius is one of these parameters and it is defined as the distance between the maximum optical power or intensity point of the optical beam and the position where the optical beam intensity is reduced by a factor of 1/exp(2). Another important parameter is the optical beam waist position along the laser beam. Over the past decades, several techniques involving moving mechanical elements have been proposed and experimentally demonstrated to determine, or “profile,” the optical beam, including measuring the optical beam radius and the optical beam profile, or shape and position of an optical beam cross section. Such profiling techniques include the use of a one dimensional (1-D) moving knife edge, see J. A. Arnaud and C. Neck, Apparatus for locating and measuring the beam-waist radius of a Gaussian laser beam, U.S. Pat. No. 3,617,755, issued Nov. 2, 1971; J. A. Arnaud, et. al., Technique for Fast Measurement of Gaussian Laser Beam Parameters, APPLIED OPTICS, Vol. 10, No. 12, 2775–2776, (December 1971), a 2-D moving knife edge, see T. F. Johnston and G. H. Williams, Apparatus for Measuring the Mode Quality of a Laser Beam, U.S. Pat. No. 5,064,284, issued Nov. 12, 1991; a translating pinhole, see P. J. Shayler, Laser Beam Distribution in the Focal Region, APPLIED OPTICS, Vol. 17, No. 17, 2673–2674, (September 1978); an encircled energy principle via a variable aperture, see P. J. Brannon, et. al., Laser Focal Spot Measurements, JOURNAL OF APPLIED PHYSICS, Vol. 46, No. 8, 3576–3579, (August 1975), a scanning slit, see M. K. Giles and E. M. Kim, Linear Systems Approach to Fiber Characterization Using Beam Profile Measurements, SPIE CONFERENCE ON FIBER OPTICS: SHORT-HAUL AND LONG-HAULMEASUREMENTS AND APPLICATIONS, Vol. 500, 67–70, (August 1984), and a rotating mirror to scan the laser beam across a photodetector, see C. P. Wang, Measuring 2-D Laser-beam Phase and Intensity Profiles: A New Technique, APPLIED OPTICS, Vol. 23, No. 9, 1399–1402, (May 1984), all of which are incorporated herein by reference.
Other optical beam profile measurements were also demonstrated including methods based on multiphoton ionization effect, see E. H. A. Granneman and M. J. van der Wiel, Laser Beam Waist Determination by Means of Multiphoton Ionization, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 46, No. 3, 332–334, (March 1975) fluorescence correlation spectroscopy, see S. M. Sorscher and M. P. Klein, Profile of a Focused Collimated Laser Beam Near the Focal Minimum Characterized by Fluorescence Correlation Spectroscopy, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 51, No. 1, 98-102, (January 1980), 2-D photodiode array, see J. T. Knudtson and K. L. Ratzlaff, Laser Beam Spatial Profile Analysis Using a Two-dimensional Photodiode Array, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 54, No. 7, 856–860, (July 1983), photo-thermal deflection, see A. Rose, Y.-X. Nie, and R. Gupta, Laser Beam Profile Measurement by Photothermal Deflection Technique, APPLIED OPTICS, Vol. 25, No. 11, 1738–1741, (June 1986), and thermographic instrument, see T. Baba, T. Arai, and A. Ono, Laser Beam Profile Measurement by a Thermographic Technique, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 57, No. 11, 2739–2742, (November 1986), all of which are incorporated herein by reference.
However, some limiting factors such as wavelength sensitivity in multiphoton ionization method, the need of a dilute solution in fluorescence correlation spectroscopy technique, and the requirement to use high power pulse laser for thermal graphic techniques are the reasons that have propelled the simpler moving mechanical element and the 2-D photodiode array approaches to being commercially dominant. These common mechanical scanning methods use scanning elements that are moved in an analog fashion and therefore require precise analog voltage control that adds cost and complexity to the optical beam profilers. Furthermore, hysteresis and motion sensitivity limits in mechanical elements reduce profiler measurement repeatability.
Hence, it would be highly desirable to digitally control the motion of the scanning elements. This type of new all-digital profiler is proposed in S. Sumriddetchkajorn and N. A. Riza, Micro-electro-mechanical system-based digitally controlled optical beam profiler, APPLIED OPTICS, Vo. 41, No. 18, (June 2002). For this innovative all-digital profiler, it would be highly desirable to reduce the digital motion limitations that limit resolution to the digital physical step of the mechanical element such as knife-edge or slit. Furthermore, it would be useful to introduce new beam profiling methods that take full advantage of the complete micromirror or pixel programmability of the 2-D pixilated device, such as a digital mirror device (DMD), leading to a higher resolution beam profile measurement. In the past, beam profilers have susceptible to optical source power fluctuations that can lead to inaccurate spatial profile measurements. Thus, an optical beam profiler insensitive to optical power fluctuations is desired.